The study objective is to test whether the administration of supplemental oxygen to infants who develop moderate retinopathy of prematurity (ROP) will result in a reduction of at least one third in the number of infants with eyes prgressing to threshold ROP. Infants receive continuous pulse oximetry saturation monitoring at one of two specified oxygen levels with weekly measurement of ROP status. The Philadelphia STOP-ROP clinical center remains active at the Children's Hospital of Philadelphia (CHOP) as part of a consortium of four intensive care units. Twenty-one patients have been enrolled in the Philadelphia center to date since recriutment began in 1994. 650 patients are projected to be enrolled nationally by April 1999 when the study will end. Enrollment of eligible patients will continue. Opthalmologists will continue ROP screening exams in the nurseries and will alert study personnel of babies who are at risk for developing ROP.